Insulating semi vaulted ceiling

Calibos

Insulating attic of a flat/apartment over a shop soon. The ceilings are semi vaulted if that's the correct term. ie. the ceilings of the flat/apartment follow the roof slope up for about a meter, then the rest/bulk of the ceiling is flat. The rafters are about 100-125mm IIRC. In other words, only 125mm thick insulation can go between the ceiling plasterboard and roof slates in these areas of vaulting near the eaves, whereas the rest of the attic space over the flat ceiling can take the normally recommended 300mm+.

My question is wether only having 125mm in those areas renders more than 125mm over the rest of the flat area of the ceiling a waste of time?

(Eaves ventilation not needed as there is no roof felt under the slates)

BryanF

Hi,
There's a few things here:
1. This method of venting the roof is questionable. Are slates in good condition? Can vent slates be installed or eaves ventilation or both? Have you discussed the project with the apartments management company?
2. The only thing limiting the sloped insulation is some plasterboard - this could be taken down, insulation fitted between the joists and below. There are various solutions to this.
3. To answer your direct question: The minimum insulation level in a certain area does not dictate the levels elsewhere. 300 mineral wool type on the flat would be a good start. At an area of reduced insulation (or at the coldest surface in a room which is often the windows) and if there is a lack of adequate ventilation condensation may occur - consider the apartments ventilarion, all thermal bridges and wall/floor insulation levels.
4. Air-tightness / vapour Barrier on the inside/warm side of the insulation is essential.

Calibos

I'm DIY'ing this but posted in this forum due to greater expertise in insulation here.

It's a pre independence mass concrete building. I will in time (when vacancy and finances coincide) deal with the floor between shop and flat in terms of sound and heat insulation and will eventually insulate the inside of the external walls with 50mm insulated plasterboard and at that point could continues the 50mm insulated plasterboard up the vaulted sections of the ceiling.

The slates are in good condition and are un felted and dot and dabbed with lime mortar. Rafters are in perfect nick after almost 100 years. There is no eaves ventilation due to the way the roof was built. ie. rafters sit on top of walls with a wooden facia board nailed to their ends around the top of the walls. ie. the facia board is flush with the underside of the edge slates. Seemingly while it's not best practice nowadays, this was normal for its time and provides enough ventilation such that cross ventilation and vent tiles were not needed. Is this not the case? That's why I thought in this case it would be fine to push insulation down this 125mm 1m long vaulted section to the back of the facia from up inside the attic.

BryanF

You need to properly ventilate if you add insulation